Exceptional Flood Research Articles

Two-Dimensional Dam-Break Flood Analysis in Data-Scarce Regions: The Case Study of Chipembe Dam, Mozambique

This paper presents the results of a modeling study of the hypothetical dam break of Chipembe dam in Mozambique. The modeling approach is based on the software Iber, a freely available dam break and two-dimensional finite volume shallow water model. The shuttle radar topography mission (SRTM) online digital elevation model (DEM) is used as main source of topographic data. Two different DEMs are considered as input for the hydraulic model: a DEM based on the original SRTM data and a hydrologically-conditioned DEM. A sensitivity analysis on the Manning roughness coefficient is performed. The results demonstrate the relevant impact of the DEM used on the predicted flood wave propagation, and a lower influence of the roughness value. The low cost modeling approach proposed in this paper can be an attractive option for modeling exceptional flood caused by dam break, when limited data and resources are available, as in the presented case. The resulting flood-inundation and hazard maps will enable the Regional Water Management Administration of Mozambique (ARA) to develop early warning systems.

Caractérisation Des Risques Hydroclimatiques Dans Le Bassin Versant De L’Ouémé A L’exutoire De Bétérou Au Bénin (Afrique De L’ouest)

In the catchment area of Ouémé with Bétérou the extreme climatic phenomena lead to risks hydroclimatic which can create, for lack of an adequate management, massive migrations of populations, ecological disasters as well as shortages of food, of energy, of essential water and other goods. The work aims to study the risks hydroclimatic in the catchment area of Ouémé to the discharge system of Bétérou. To achieve this goal, from the climatological data (height daily and monthly of rains, temperatures, ETP) over 1965 to 2012 period are obtained at the National Direction of the Meteorology (DNM) of Cotonou. The hydrological data made up of the daily outputs of the Ouémé river to the discharge system of Bétérou, over 1965-2012 period are extracted from the data base of DGWater. The diagnostic methods of the climate used are primarily statistical and relate to the frequential analysis and the calculation of the indices of dryness. The analysis of the results shows that in the catchment area of Ouémé with Bétérou analyzes compared of the three indices of weather dryness (Decile, RDI and the SPI), that the basin known over the period of 1965 to 1990 three years very dry (1981, 1982 and 1987) and a year close to the extreme dryness (1983). It also arises that index PDSI made it possible to identify the various periods of dryness recorded in the middle In the basin, the recurrences of the rains and maximum us flows are also 2, 5, 10, 20, 50, 100 and 200 years and make it possible to say that the area can be in prey of exceptional flood which can likely involve floods which will not be without consequences on the local development.

Exceptional floods in the Prut basin, Romania, in the context of heavy rains in the summer of 2010

Abstract. The year 2010 was characterized by devastating flooding in central and eastern Europe, including Romania, the Czech Republic, Slovakia, and Bosnia-Herzegovina. This study focuses on floods that occurred during the summer of 2010 in the Prut River basin, which has a high percentage of hydrotechnical infrastructure. Strong floods occurred in eastern Romania on the Prut River, which borders the Republic of Moldova and Ukraine, and the Siret River. Atmospheric instability from 21 June to 1 July 2010 caused remarkable amounts of rain, with rates of 51.2 mm/50 min and 42.0 mm/30 min. In the middle Prut basin, there are numerous ponds that help mitigate floods as well as provide water for animals, irrigation, and so forth. The peak discharge of the Prut River during the summer of 2010 was 2310 m3 s−1 at the Rădăuţi-Prut gauging station. High discharges were also recorded on downstream tributaries, including the Baseu, Jijia, and Miletin. High discharges downstream occurred because of water from the middle basin and the backwater from the Danube (a historic discharge of 16 300 m3 s−1). The floods that occurred in the Prut basin in the summer of 2010 could not be controlled completely because the discharges far exceeded foreseen values.

Metals impact into the Paranaguá Estuarine Complex (Brazil) during the exceptional flood of 2011

Abstract Particulate and dissolved metal concentrations were determined after the largest flood in the last 30 years on the east-west axis of the Paranaguá Estuarine Complex (PEC) and compared to the those of the dry period at two stations. Results confirmed that the flood greatly affected riverine outflows and the behavior of metals in the PEC. In particular, a sharp decrease in salinity was followed by extremely high SPM concentrations leading to a decrease in DO concentrations at both stations. For the dissolved phase, ANOSIM analysis showed a significant dissimilarity at each station between the sampled periods, whereas for the particulate phase this dissimilarity was found only for the samplings taken at the Antonina Station. KD values suggested dissolved Cu behavior was related to the presence of organic complexes and dissolved Mn had sediment resuspension of redox sediments and or/pore water injection as sources. Metal concentrations were lower than in polluted estuaries, though high enrichment factors found after the flood pointed to the influence of anthropogenic sources. In conclusion, the flood's influence was more evident at the Antonina Station, due to its location in the upper estuary, whereas in Paranaguá a high SPM content with low metal concentration was found, following the common pattern generally found in other marine systems subject to heavy rainfall events.

Semi-2D modeling of river morphological changes caused by exceptional flooding

This paper presents the application of the model, MHYSER 1.0 (Model for the HYdraulics of SEdiments in Rivers, version 1.0), to study river morphological changes due to exceptional flooding. MHYSER 1.0 is a semi-two-dimensional model using the stream tubes concept to achieve lateral variations of velocity, flow stresses and sediment transport rate. Each stream tube has the same conveyance as the other ones. In MHYSER 1.0, the uncoupled approach is used to solve the set of conservation equations’. After the backwater calculation, the river is divided in a finite number of stream tubes of equal conveyances. The sediment routing and bed adjustments calculations are accomplished separately along each stream tube taking into account lateral mass exchanges. MHYSER 1.0 is applied to the Ha! Ha! River (Quebec, Canada), a tributary of the Saguenay River, for the 1996 “downpour“. The results show good correlation of with collected data. This application shows the capabilities of this model and predicts its promising role in solving complex real engineering cases.

Evidence of flash floods in Precambrian gravel dominated ephemeral river deposits

Fluvial strata at the base of the Whyte Inlet Formation on Baffin Island, to the west of Sikosak Bay, are predominantly boulder and cobble bearing large pebble conglomerates of braided fluvial origin. Local development of narrow sinuous channels, possibly within the thalwegs of an initially braided bedrock confined system, is indicated by the presence of lateral accretion surfaces, some of which host isolated sub-vertically oriented boulders. These larger boulders were probably emplaced during exceptional flood events involving either hyper-concentrated flows or dilute debris flows, with velocities in the order of 2.2m/s. Isolated ridges of boulders and cobbles are perched on the upper parts of lateral accretion surfaces in mixed sandy-gravelly fluvial intervals. These boulder berms developed down stream from channel bends or bedrock constrictions in response to flow expansion during flash floods, with estimated peak discharge of about 1.4m/s. Associated sandstones on lateral accretion surfaces show evidence of deposition under both upper and lower flow conditions. Similar boulder and cobble berms of this type are known from modern ephemeral and highly seasonal fluvial systems in a wide range of climatic settings, and are a clear indication that highly variable to catastrophic discharge events affected the rivers responsible for deposition of these conglomerates.

Nitrite consumption and associated isotope changes during a river flood event

Abstract. In oceans, estuaries, and rivers, nitrification is an important nitrate source, and stable isotopes of nitrate are often used to investigate recycling processes (e.g. remineralisation, nitrification) in the water column. Nitrification is a two-step process, where ammonia is oxidised via nitrite to nitrate. Nitrite usually does not accumulate in natural environments, which makes it difficult to study the single isotope effect of ammonia oxidation or nitrite oxidation in natural systems. However, during an exceptional flood in the Elbe River in June 2013, we found a unique co-occurrence of ammonium, nitrite, and nitrate in the water column, returning towards normal summer conditions within 1 week. Over the course of the flood, we analysed the evolution of δ15N–NH4+ and δ15N–NO2− in the Elbe River. In concert with changes in suspended particulate matter (SPM) and δ15N SPM, as well as nitrate concentration, δ15N–NO3− and δ18O–NO3−, we calculated apparent isotope effects during net nitrite and nitrate consumption. During the flood event, > 97 % of total reactive nitrogen was nitrate, which was leached from the catchment area and appeared to be subject to assimilation. Ammonium and nitrite concentrations increased to 3.4 and 4.4 µmol L−1, respectively, likely due to remineralisation, nitrification, and denitrification in the water column. δ15N–NH4+ values increased up to 12 ‰, and δ15N–NO2− ranged from −8.0 to −14.2 ‰. Based on this, we calculated an apparent isotope effect 15ε of −10.0 ± 0.1 ‰ during net nitrite consumption, as well as an isotope effect 15ε of −4.0 ± 0.1 ‰ and 18ε of −5.3 ± 0.1 ‰ during net nitrate consumption. On the basis of the observed nitrite isotope changes, we evaluated different nitrite uptake processes in a simple box model. We found that a regime of combined riparian denitrification and 22 to 36 % nitrification fits best with measured data for the nitrite concentration decrease and isotope increase.

Geomorphic change and sediment transport during a small artificial flood in a transformed post-dam delta: The Colorado River delta, United States and Mexico

The Colorado River delta is a dramatically transformed landscape. Major changes to river hydrology and morpho-dynamics began following completion of Hoover Dam in 1936. Today, the Colorado River has an intermittent and/or ephemeral channel in much of its former delta. Initial incision of the river channel in the upstream ∼50km of the delta occurred in the early 1940s in response to spillway releases from Hoover Dam under conditions of drastically reduced sediment supply. A period of relative quiescence followed, until the filling of upstream reservoirs precipitated a resurgence of flows to the delta in the 1980s and 1990s. Flow releases during extreme upper basin snowmelt in the 1980s, flood flows from the Gila River basin in 1993, and a series of ever-decreasing peak flows in the late 1990s and early 2000s further incised the upstream channel and caused considerable channel migration throughout the river corridor. These variable magnitude post-dam floods shaped the modern river geomorphology. In 2014, an experimental pulse-flow release aimed at rejuvenating the riparian ecosystem and understanding hydrologic dynamics flowed more than 100km through the length of the delta’s river corridor. This small artificial flood caused localized meter-scale scour and fill of the streambed, but did not cause further incision or significant bank erosion because of its small magnitude. Suspended-sand-transport rates were initially relatively high immediately downstream from the Morelos Dam release point, but decreasing discharge from infiltration losses combined with channel widening downstream caused a rapid downstream reduction in suspended-sand-transport rates. A zone of enhanced transport occurred downstream from the southern U.S.-Mexico border where gradient increased, but effectively no geomorphic change occurred beyond a point 65km downstream from Morelos Dam. Thus, while the pulse flow connected with the modern estuary, deltaic sedimentary processes were not restored, and relatively few new open surfaces were created for establishment of native riparian vegetation. Because water in the Colorado River basin is completely allocated, exceptional floods from the Gila River basin are the most likely mechanism for major changes to delta geomorphology for the foreseeable future.

Temporal clustering of floods in Germany: Do flood-rich and flood-poor periods exist?

The repeated occurrence of exceptional floods within a few years, such as the Rhine floods in 1993 and 1995 and the Elbe and Danube floods in 2002 and 2013, suggests that floods in Central Europe may be organized in flood-rich and flood-poor periods. This hypothesis is studied by testing the significance of temporal clustering in flood occurrence (peak-over-threshold) time series for 68 catchments across Germany for the period 1932–2005. To assess the robustness of the results, different methods are used: Firstly, the index of dispersion, which quantifies the departure from a homogeneous Poisson process, is investigated. Further, the time-variation of the flood occurrence rate is derived by non-parametric kernel implementation and the significance of clustering is evaluated via parametric and non-parametric tests. Although the methods give consistent overall results, the specific results differ considerably. Hence, we recommend applying different methods when investigating flood clustering. For flood estimation and risk management, it is of relevance to understand whether clustering changes with flood severity and time scale. To this end, clustering is assessed for different thresholds and time scales. It is found that the majority of catchments show temporal clustering at the 5% significance level for low thresholds and time scales of one to a few years. However, clustering decreases substantially with increasing threshold and time scale. We hypothesize that flood clustering in Germany is mainly caused by catchment memory effects along with intra- to inter-annual climate variability, and that decadal climate variability plays a minor role.

Morphosedimentary features from a major flood on a small, lower-sinuosity, single-thread river: The unknown quantity of overbank deposition, historical-change context, and comparisons with a multichannel river

This study details overbank sedimentation on a single-thread, bedload stream, the Little Blue River (southeastern Nebraska and northeastern Kansas, USA), which experienced exceptional floods during May–June 2015. Downvalley elongate splays left deposits as thick as 1.2m and attained areas as great as 156ha. Splays consisted of fields of large, gravel-covered dunes 0.1 to 0.3m in height and extensive sand sheets with current ripples or current ripples atop low-relief, long-wavelength dunes. Erosion occurred chiefly in the heads of splays. Levee-floodplain channel complexes were more architecturally complicated, with channel–channel-fringing deposits deposited within ~110m of the banks of the channel. These complexes consisted of two amalgamated components: (1) sediment shadows (vegetation-induced sedimentary structures), as long as 6m and as high as 0.6m at their heads, which were produced by the interaction of direct overbank flow with rooted trees; and (2) indistinct, flat-topped bars (some of which were built around eroded remnants of sediment shadows) shaped chiefly by downvalley flood flow. Changes on in-channel bars, the erosion of bank crevasses as large as 70m in width, and the deposition of large woody debris also occurred during the floods.The earliest aerial photographs (1938) of the study area, taken after multiple low-flow years, show no evidence for recent overbank deposition. Increases in channel-belt area, the number of bars, and the sizes of bars in the Little Blue River took place after 1938, and most rapidly during 1938 to 1950, when annual discharge was increasing. Annual discharge has decreased since ca. 1990, and yet there has been a long-term increase in annual peak from since the 1930s. We surmise that increasing annual peak flows have led to larger floods and, since 1938, serial overbank deposition, as well a wider channel and numerous, large compound bars.

Controls, budgets and variability of riverine sediment fluxes to the Gulf of Lions (NW Mediterranean Sea)

The present study investigates the spatio-temporal variability of riverine sediment fluxes to the Gulf of Lions, one of the most extensive shelf regions in the Mediterranean Sea. Small coastal rivers compete here with the Rhone River, nowadays the largest Mediterranean river in terms of water discharge. Our scientific objectives were to investigate the major controls of riverine sediment yields (SY) in this area and to quantify the role of the small coastal rivers, largely ignored in previous studies, in the total sediment budgets. Another objective concerned the source identification of the Rhone sediments with regard to the major tributary contributions, and to test whether the sediment fluxes are in equilibrium in the basin. For the calculation of representative long-term fluxes, we used a Simplified Rating Curve Approach (SiRCA) which could be validated by high resolution monitoring and existing literature data. An overall of 13 drainage basins could be distinguished, covering about 86% of the study area (6 coastal rivers, the Rhone River, and 6 of its tributaries). Representative SY range from 19 to 151tkm−2yr−1 in the investigated drainage basins. Despite their smaller basin areas and more torrential discharge regimes, SY of the coastal rivers were generally lower compared to SY of the Rhone River and its tributaries. Confrontation with the lithological, morphological and hydroclimatic basin characteristics indicate that lithology exerts the dominant control on SY in the study region. In particular, the existence of erodible sedimentary rocks in the headwater regions yields high SY. Peak values of 135 and 151tkm−2yr−1 were observed for the Isere and Durance tributaries of the Rhone River, where badlands exist. The coastal rivers contribute on average only to slightly more than 5% of the long-term sediment inputs to the Gulf of Lions. During individual years however, their contribution can strongly increase (up to 27% in 2011). Their contribution is generally important during late spring and autumn, when flash-floods are frequent (up to 90% of the monthly sediment discharge). Summing the various tributary contributions of the Rhone River produces a sediment budget that is close to the sediment export at the river mouth. This indicates that at spatial scales, the average sediment fluxes are equilibrated in the basin. About 40% of the Rhone sediments originate from the Isere and Durance tributaries. However, fully closure of the budgets requires high SY in the lowermost basin parts. Omitting in our calculations only a few years with exceptional flooding considerably reduce these values. Trend analyses show at the same time that the sediment fluxes from the Saone and Isere tributaries decreased during the study period. At temporal scales, sediment transport from upstream to downstream was therefore not in equilibrium and the remobilisation of older sediments from the downstream basin parts may have been important.

Exceptional floods in small basins in North-Western Romania and the induced effects – Bârsău River, Maramureș county

Exceptional floods in small basins in North-Western Romania and the induced effects – Bârsău River, Maramureș county

Continuous earlywood vessels chronologies in floodplain ring-porous species can improve dendrohydrological reconstructions of spring high flows and flood levels

Plants respond to environmental stimuli through changes in growth and development. Characteristics of wood cells such as the cross-sectional area of vessel elements (hereafter referred to as vessels) may store information about environmental factors present at the time of vessel differentiation. The analysis of vessel characteristics therefore offers a different time resolution than annual ring width because vessels in tree rings differentiate within days to a few weeks. Little research has been conducted on the sensitivity of earlywood vessels in ring-porous species in response to flooding. The general objectives of this study were to determine the plasticity of earlywood vessel to high flows and spring flooding in floodplain black ash (Fraxinus nigra Marsh.) trees and to assess the utility of developing continuous earlywood vessel chronologies in dendrohydrological reconstruction. In contrast, most dendrohydrological studies until now have mainly used vessel anomalies (flood rings) as discrete variables to identify exceptional flood events. The study area is located in the boreal region of northwestern Québec. Vessel and ring-width chronologies were generated from F. nigra trees growing on the floodplain of Lake Duparquet. Spring discharge had among all hydro-climatic variables the strongest impact on vessel formation and this signal was coherent spatially and in the frequency domain. The mean vessel area chronology was significantly and negatively correlated to discharge and both the linearity and the strength of this association were unique. In floodplain F. nigra trees, spring flooding promoted the formation of more abundant but smaller earlywood vessels. Earlywood vessels chronologies were also significantly associated with other hydrological indicators like Lake Duparquet’s ice break-up date and both ice-scar frequency and height chronologies. These significant relationships stress the utility of developing continuous vessels chronologies for hydrological reconstructions prior to instrumental data. Continuous earlywood vessel chronologies may also be useful in determining the impact of altered hydrological regime in floodplain habitat regulated by spring floods. Future research should involve quantifying the impact of high flows and flooding on other cell constituents and also determining the plasticity and utility of continuous anatomical series in floodplain diffuse-porous species.

ORRION: A specific information sharing tool to (re)build a “flood risk culture” in the Rhine Graben (France - Germany)

When is led a prevention initiative concerning flood risk, the recall of old events allows just as much to act as pedagogy to legitimize the initiative. So as it is written in the information leaflet on the “Plan de Prevention des Risques d’Inondation” (PPRI):«Alsace did not know any more exceptional floods since 1910. In almost a century, the flood risk thus naturally faded of our memory”. A paradoxical assertion because the floods of 1910 are not taken into account for risk mapping. Only the recent floods (1983, 1990) are used as benchmarks. We were able to show [1] that, although legitimate as reference events, the floods of 1910 and many others had become manifestly underestimated for multiple reasons connected to the history of Alsace, or due to disorganization of the archives. Worse, these riks unawareness is shared by all the actors, causing problems of acceptance of the risk and their management, and an additional vulnerability in terms of population behavior before, during and after the crises. This is the reason why an online participative database (ORRION) has been created. the objective of this database is to inform and to share the information about floods history, in the particular context of a cross-border territory (France, Germany, Switzerland). This tool has been immediately involved into risks prevention actions (PAPI, PPRI).

The role of log jams and exceptional flood events in mobilizing coarse particulate organic matter in a steep headwater stream

Abstract. Coarse particulate organic matter (CPOM) fulfills important functions in the physical and ecological system of a stream. CPOM delivery to and export from the stream has implications for the stream's morphology and sediment transport capacity as well as the energy budget and food availability. Export rates of CPOM from mountain catchments have been observed to strongly increase with rising discharge, but the mechanism leading to this strong relationship is unclear. Here, we show that log jams in the Erlenbach, a steep headwater stream in the Swiss Prealps, are an effective barrier for the transport of CPOM pieces, and thus become sites of storage of large quantities of material over time. Exceptional discharge events with return periods exceeding 20 years play a dual role in CPOM transport in the Erlenbach. First, they appear to destroy existing log jams, releasing the stored material (wood and sediment). Second, they intensify channel–hillslope coupling, thereby recruiting new logs to the channel, around which new jams can form. This allows for the formulation of a new, fully episodic end-member in a four-end-member model of CPOM dynamics of steep mountain streams based on wood delivery and export.

Exceptional floods on a developed river: case study for the Bistrita River from the Eastern Carpathians (Romania)

The geographical position of the Bistrita River favours a rich liquid run-off and great hydrotechnical potential. The Bistrita drainage basin is strongly developed from the hydrotechnical point of view. Within the drainage basin, numerous dams and storage reservoirs are installed with the purpose of producing electricity and reducing flood hazards. However, these structures could not stop floods at the drainage basin level. The most powerful floods from Bistrita River basin and the role in flood control of each of the storages are analysed in this paper. The hydrological data have been collected from the monitoring network, where the researchers elaborating this study are performing their daily activity. The most powerful floods in the last century took place in 1970, 1991, 2005 and 2010. An intensification of the floods can be noticed after the year 1990, a period which coincides with large-scale deforestation of the small drainage basins. Disastrous floods occurred as a consequence of summer heavy rains, with amounts of 100–200 mm within 24 h. The most powerful floods were recorded in 2005 and registered discharges up to 1,200 m3/s. Izvoru Muntelui reservoir, the biggest in the system, can take most of the waters in excess. The reservoirs located downstream are silting up, and the supplementary flows cannot be absorbed entirely by storage. The exact knowledge of flooding phenomena requires studies that can prevent and diminish the damages caused by extreme hydrological events.

Analysis of Lake Champlain/Richelieu River’s historical 2011 flood

Lake Champlain and its natural outlet, the Richelieu River, underwent an historical flooding event in 2011, exceeding by a wide margin all other extreme events in over 100 years of monitoring records. In Québec, hundreds of homes were flooded for up to 2 months, requiring waves of evacuation and military involvement to support the effort. This paper examines the causes of this exceptional flood, performs a statistical analysis of important variables and compares the 2011 event to three other large floods. The results show that the return period of the 2011 spring flood exceeded 700 years and was caused by the combination of extreme precipitation over the spring season (500 years recurrence) and an important snowpack (15 years recurrence). Hydrological modeling of past extreme events over the Lake Champlain watershed indicates that liquid precipitation during snowmelt is the key factor in triggering large spring floods. The combination of the 2011 April and May precipitation with the large 2008 snowpack in a hypothetical meteorological scenario shows simulated discharges and lake levels much higher than the ones seen in 2011.

What made the June 2013 flood in Germany an exceptional event? A hydro-meteorological evaluation

Abstract. The summer flood of 2013 set a new record for large-scale floods in Germany for at least the last 60 years. In this paper we analyse the key hydro-meteorological factors using extreme value statistics as well as aggregated severity indices. For the long-term classification of the recent flood we draw comparisons to a set of past large-scale flood events in Germany, notably the high-impact summer floods from August 2002 and July 1954. Our analysis shows that the combination of extreme initial wetness at the national scale – caused by a pronounced precipitation anomaly in the month of May 2013 – and strong, but not extraordinary event precipitation were the key drivers for this exceptional flood event. This provides additional insights into the importance of catchment wetness for high return period floods on a large scale. The database compiled and the methodological developments provide a consistent framework for the rapid evaluation of future floods.

WRF model sensitivity to choice of parameterization: a study of the ‘York Flood 1999’

Numerical weather modelling has gained considerable attention in the field of hydrology especially in un-gauged catchments and in conjunction with distributed models. As a consequence, the accuracy with which these models represent precipitation, sub-grid-scale processes and exceptional events has become of considerable concern to the hydrological community. This paper presents sensitivity analyses for the Weather Research Forecast (WRF) model with respect to the choice of physical parameterization schemes (both cumulus parameterisation (CPSs) and microphysics parameterization schemes (MPSs)) used to represent the ‘1999 York Flood’ event, which occurred over North Yorkshire, UK, 1st–14th March 1999. The study assessed four CPSs (Kain–Fritsch (KF2), Betts–Miller–Janjic (BMJ), Grell–Devenyi ensemble (GD) and the old Kain–Fritsch (KF1)) and four MPSs (Kessler, Lin et al., WRF single-moment 3-class (WSM3) and WRF single-moment 5-class (WSM5)] with respect to their influence on modelled rainfall. The study suggests that the BMJ scheme may be a better cumulus parameterization choice for the study region, giving a consistently better performance than other three CPSs, though there are suggestions of underestimation. The WSM3 was identified as the best MPSs and a combined WSM3/BMJ model setup produced realistic estimates of precipitation quantities for this exceptional flood event. This study analysed spatial variability in WRF performance through categorical indices, including POD, FBI, FAR and CSI during York Flood 1999 under various model settings. Moreover, the WRF model was good at predicting high-intensity rare events over the Yorkshire region, suggesting it has potential for operational use.

Freedom space for rivers: a sustainable management approach to enhance river resilience.

River systems are increasingly under stress and pressure from agriculture and urbanization in riparian zones, resulting in frequent engineering interventions such as bank stabilization or flood protection. This study provides guidelines for a more sustainable approach to river management based on hydrogeomorphology concepts applied to three contrasted rivers in Quebec (Canada). Mobility and flooding spaces are determined for the three rivers, and three levels of "freedom space" are subsequently defined based on the combination of the two spaces. The first level of freedom space includes very frequently flooded and highly mobile zones over the next 50 years, as well as riparian wetlands. It provides the minimum space for both fluvial and ecological functionality of the river system. On average for the three studied sites, this minimum space was approximately 1.7 times the channel width, but this minimum space corresponds to a highly variable width which must be determined from a thorough hydrogeomorphic assessment and cannot be predicted using a representative average. The second level includes space for floods of larger magnitude and provides for meanders to migrate freely over a longer time period. The last level of freedom space represents exceptional flood zones. We propose the freedom space concept to be implemented in current river management legislation because it promotes a sustainable way to manage river systems, and it increases their resilience to climate and land use changes in comparison with traditional river management approaches which are based on frequent and spatially restricted interventions.